A spiral (sometimes called a spiral loop or just loop) is a technique employed by railways to ascend steep hills.

A railway spiral rises on a steady curve until it has completed a loop, passing over itself as it gains height, allowing the railway to gain vertical elevation in a relatively short horizontal distance. It is an alternative to a zig-zag, and avoids the need for the trains to stop and reverse direction while ascending. If the train is of sufficient length, it is often possible to view the train looping above itself.

A spiral loop should not be confused with the transition spiral or spiral easement used to provide a transition from a tangent into a horizontal circular curve. Spiral easement is used to avoid abrupt changes in the sideward acceleration experienced by a railway vehicle and the passengers in the vehicle approaching the horizontal circular curve and to prevent abrupt forces and discomfort. These curve layouts are also used in highway engineering.

Spirals are not generally needed on tramways (street running or reserved track) or light rail lines because trams traditionally have all wheels powered, giving the ability to climb much steeper direct gradients than railways. In addition, the space consumed by a spiral is usually not available in an urban environment.

A similar feature to railway spirals in road design is the pigtail bridge.

On a railway climbing at a gradient of 1 in 40 (2.5%, or 25 m per km) a 360-degree spiral at 350 m radius will add 2200 m to the forward journey and 55 m to the vertical climb. Unless the topography has a suitably shaped hill, the spiral is likely to include a tunnel, increasing construction costs and creating problems if steam locomotives are employed. If a convenient side valley is available, then a horseshoe curve may be possible.

The spiral needs to climb about 6 m in order to bridge itself. With steam locomotives and to a lesser extent with diesel locomotives, the gradient in any long tunnel(s) should be less than the ruling grade to avoid problems with fumes and dampness causing the driving wheels to slip.

Geumdae 2nd Tunnel – between Geumgyo station and Chiak station on Jungang Line – one loop, single track. New double track tunnel is under construction to replace loop tunnel.

Daegang Tunnel – between Danseong station and Jungnyeong station on the Jungang Line – one loop, single track.

Hambaek 1st Tunnel – between Hambaek station and Jodong station on the Hambaek Line – one loop, single track.

Solan Tunnel – between East Baeksan station and Dogye station on the Yeongdong Line – one loop, single track but double track is installed on the middle of the tunnel; for both trains crossing each other. It will be opened in 2009.